OBJECTIVE This experimental project was designed to evaluate: a) the capacity of oxytetracycline to induce microvesicle steatosis in rat liver when administered over long time periods; b) whether female rats are more susceptible to this substance, and c) the possible ultrastructural alterations and their relation to the mechanisms of steatosis. METHODS Sixty-two Wistar rats (31 males and 31 females) were distributed into six groups, two control groups and four experimental groups. The experiment lasted three months. Blood and hepatic tissue samples were extracted under anesthesia for morphologic study (optical and electron microscopy). RESULTS Steatosis was of the microvesicular type with mainly periportal distribution. Steatosis developed in the treated groups and the degree was significantly greater in the females (p = 0.004). No relationship was found with dose. Ultrastructural study revealed microsome dilation in all experimental groups, with no differences according to sex. Despite the steatosis, no proliferation of peroxisomes or mitochondrial alterations were observed. CONCLUSIONS Oxytetracycline produced predominantly periportal microvesicular hepatic steatosis, appearing mostly in the females. As a possible mechanism for tetracycline-induced steatosis, we postulate a decrease in mitochondrial, peroxisome and microsome function as a result of protein synthesis inhibition in these cell compartments.